Transmission of HIV-1 is a rare event that involves extreme, non-random selection of as few as one founder genotype out of as many as 100 million genotypes in the infected donor. The goal of this research proposal is to understand why transmission is so selective, and what biological properties define the rare, highly transmissible virus. The use of infectious molecular clones of founder viruses from 4 subtype C primary infections and 1 subtype B infection provides a key resource to distinguish the few highly transmissible viruses from the many non-transmissible viruses. The research proposal has two specific aims. The first is to model HIV transmission in vitro using transwell cultures where virus must cross an intact epithelial barrier to reach target cells. Both the conditions of virus addition and the available target cells will be varied to mimic the natural sites of mucosal transmission and the resident CD4+ T cell targets. The hypothesis under test is that a highly transmissible virus must be able to both cross the epithelial cell barrier efficiently and infect the first available target cell efficiently, and that these two properties can be modeled in vitro to distinguish readily transmissible viruses from poorly transmissible viruses. In the second specific aim, the results of the in vitro model will be put to the test by creating a simian-human immunodeficiency virus envelope chimera with the best transmitted HIV-1 envelope, and using the resulting SHIV for vaginal challenge studies in rhesus macaques. In both in vitro and in vivo studies, extensive analysis of the genotypic and biological properties of highly transmissible versus poorly transmissible viruses will allow the definition of the characteristics that define transmission success. The end product of this research program will thus be a better understanding of HIV-1 transmission, and one or more SHIV chimeric viruses that reflect founder viruses rather than late-stage isolates, and will provide a much more relevant reagent for preclinical prevention studies.